Lithium containing transition metal sulfide compounds

ABSTRACT

The present invention provides composition comprising at least one lithium-containing transition metal sulfide and carbon, wherein particles of the carbon are dispersed at the microscopic level on individual particles of the lithium-containing transition metal sulfide.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a United States national phase application under 35 U.S.C. §371 of International Patent Application No. PCT/GB2009/051312 filed on Oct. 6, 2009, and claims the benefit of Great Britain Patent Application No. 0818758.5 filed on Oct. 14, 2008, both of which are herein incorporated in their entirety by reference. The International Application was published as International Publication No. WO 2010/043884 on Apr. 22, 2010.

FIELD

The present invention relates to compositions comprising lithium-containing transition metal sulfide compounds, the use of such compositions in a cell or battery and in electrode materials for lithium ion cells or batteries, and the use of such cells or batteries in commercial products.

BACKGROUND

Lithium ion batteries are secondary batteries that comprise an anode (negative electrode), a cathode (positive electrode) and an electrolyte material. They operate by the transfer of lithium ions between the anode and the cathode, and they are not to be confused with lithium batteries, which are characterised by containing metallic lithium. Lithium ion batteries are currently the most commonly used type of rechargeable battery and typically the anode comprises an insertion material, for example, carbon in the form of coke or graphite. An electroactive couple is formed using a cathode that comprises a lithium-containing insertion material. Typical lithium-containing insertion materials are lithium cobalt oxide (LiCoO₂), lithium nickel oxide (LiNiO₂) and lithium manganese oxide (LiMn₂O₄). In its initial condition, this type of cell is uncharged; therefore, to deliver electrochemical energy the cell must be charged to transfer lithium to the anode from the lithium-containing cathode. Upon discharge, the lithium ions are transferred from the anode back to the cathode. Subsequent charging and discharging operations transfer the lithium ions back and forth between the cathode and the anode over the life of the battery. A review of the recent developments and likely advantages of lithium rechargeable batteries is provided by Tsutomu Ohzuku and Ralph Brodd in Journal of Power Sources 2007.06.154.

Unfortunately, lithium cobalt oxide is a relatively expensive material and the nickel compounds are difficult to synthesize. Not only that, cathodes made from lithium cobalt oxide and lithium nickel oxide suffer from the disadvantage that the charge capacity of a cell is significantly less than its theoretical capacity. The reason for this is that less than 1 atomic unit of lithium engages in the electrochemical reaction. Moreover, the initial capacity is reduced during the initial charging operation and still further reduced during each charging cycle. Prior art U.S. Pat. No. 4,828,834 attempts to control capacity loss through the use of a cathode mainly composed of LiMn₂O₄. U.S. Pat. No. 5,910,382 on the other hand, describes another approach using lithium-mixed metal materials such as LiMPO₄ where M is at least one first row transition metal. Preferred compounds include LiFePO₄, LiMnPO₄, LiCoPO₄, and LiNiPO₄ and mixed transition metal compounds such as Li_(1-2x)Fe_(1-x)Ti_(x)PO₄ or Li_(1-2x)Fe_(1-x)Mn_(x)PO₄ where 0<x<1.

The use of lithium ion rechargeable batteries is limited by the prohibitive cost of providing the lithium electrode material, particularly in the case of lithium cobalt oxide. Consequently, current commercialisation is restricted to premium applications such as portable computers and mobile telephones. However, it would be highly desirable to gain access to wider markets, for example, the powering of electric vehicles and work has been ongoing in recent years to produce materials that maintain the high performance of lithium ion batteries, but which at the same time, are much cheaper to produce. To achieve this goal, it has been suggested, for example, in JP Kokai No 10208782 and Solid State Ionics 117 (1999) 273-276), that sulfides may be used in place of oxides as cathode materials. Although the use of many sulfides achieves less voltage measured against lithium of the corresponding oxides, the capacity of some sulfide-based cathodes, measured in milliampere hours per gram, can be as much as about 3 times greater. Based on this, some sulfide-based cathodes achieve an overall advantage of about 1.5 times in terms of cathode energy density for batteries measured against a lithium metal anode, as compared against their oxide counterparts, and this makes the use of these sulfides a very attractive proposition. For example, in the case of lithium iron sulfide a theoretical capacity of 400 mAhg⁻¹ may be obtained with an average operating voltage of 2.2V versus a lithium metal anode.

Thus, lithium-containing transition metal sulfides will be a convenient substitute material for the lithium metal oxides described above, with lithium iron sulfide already described in the patent literature, for example in U.S. Pat. No. 7,018,603, to be a useful cathode material in secondary cells. The commercialisation of lithium-containing transition metal sulfides will depend largely on their cost of production. Taking lithium iron sulfide as a specific example, the conventional process for making this material is via a solid-state reaction in which lithium sulfide (Li₂S) and ferrous sulfide (FeS) are intimately mixed together and heated under an inert atmosphere at a temperature of about 800° C. The starting materials, ferrous sulfide (FeS) and iron disulfide (FeS₂), are relatively inexpensive as they are naturally occurring materials, and are dug out of the ground. However, a notable disadvantage of the reaction process is that the other starting material, Li₂S, is not only expensive but also highly moisture sensitive. The latter problem in particular has obvious implications for the complexity, and therefore the cost, of storing and handling the starting material, especially for large-scale commercial production. In addition, the kinetics of this reaction are reported in U.S. Pat. No. 7,018,603 to be very slow and it can apparently take up to one month to complete the reaction. Thus, this route is believed to be highly unfavourable in terms of energy costs and not commercially viable for the production of electrode materials.

As an alternative route for making lithium-containing transition metal sulfides, U.S. Pat. No. 7,018,603 discloses reacting a transition metal sulfide such as FeS with lithium sulfide in a reaction medium comprising molten salt or a mixture of molten salts at high temperature (temperatures of 450° C. to 700° C. are exemplified). The preferred molten salts are lithium halides. Whilst this reaction proceeds at a good rate there are still several issues that make it less than ideal. Firstly, the fact it uses Li₂S as a starting material leads to the handling and storage problems described above. Secondly, it is very difficult to separate the reaction medium (molten lithium halide used in 1.5 molar excess) from the desired reaction product by means other than solvent extraction, which is expensive. Further, even after rigorous purification as much as 8% of the reaction medium salt is still present in the reaction product. This level of impurity is detrimental to the charge capacity per gram of lithium iron sulfide.

Given the problems associated with the above synthetic routes to make lithium transition metal sulfides, it is highly desirable to find further alternative routes which rely on inexpensive and non-moisture sensitive starting materials, and which involve a simple, energy efficient reaction method to produce a clean product.

Thus in another patent application (hereafter, “the first patent application”), which claims the same priority as the present application, the applicant has claimed a method of producing a lithium-containing transition metal sulfide characterised in that it comprises the steps of a) mixing at least one transition metal sulfide with a lithium-containing compound; b) heating the resultant mixture to effect evolution of sulfur from the transition metal sulfide; and c) allowing sufficient time for the resulting lithium-transition metal sulfide to form, wherein sulfur is retained within the reaction vessel for reaction with the lithium-containing compound and further wherein the lithium-containing compound is selected from one or more of lithium oxide, lithium carbonate, anhydrous lithium hydroxide, lithium hydroxide monohydrate, lithium oxalate and lithium nitrate and any material that is a precursor for any of these lithium-containing compounds during the heating step. The transition metal sulfide made by the method of the first patent application is of the formula Li_(2-x-y) A_(y) Fe_(1-z) M_(z)S₂ where x=0 to 1.5, preferably x=0 to 1, further preferably x=0 to 0.5 and particularly preferably x=0 to 0.3; y=0 to 1; z=0 to 1, A is selected from one or more of silver (Ag), sodium (Na), copper (Cu(I)) and potassium (K) and M is a generic representation for one or more transition metals. The preferred lithium-containing compounds comprise lithium oxide and/or one or more materials that is a precursor for lithium oxide. The precursor materials for lithium oxide decompose to give lithium oxide during the heating step of the method of the first patent application. Also during the heating step, it is advantageous that the transition metal sulfide decomposes to release sulfur, which in turn reacts with the lithium-containing compound to form in situ the transition metal, sulfur and lithium-containing compounds required to produce the lithium-containing transition metal sulfide.

The applicant has found that to improve the yield and efficiency of the reaction it is highly advantageous if the method of the invention according to the first patent application is conducted under a non-oxidizing atmosphere and/or reducing conditions. Many of the well-used non-oxidizing atmosphere and/or reducing conditions may be employed. However, preferred examples involve one or more reducing gases, such as carbon monoxide, hydrogen, reforming gas (mixture of hydrogen and nitrogen), hydrogen sulfide, methane and other gaseous alkanes. One or more reducing agents such as carbon may also be used either alone or in combination with a reducing gas and/or non-oxidizing atmosphere. In the method according to the first patent application, it is highly preferred that the reducing conditions do not reduce the oxidation state of the transition metal ion.

The ideal reaction temperature used in the method of the first patent application is that which, on the one hand, is sufficient to cause the transition metal sulfide to decompose to evolve sulfur so that it is available and at the correct temperature for reaction with the lithium-containing compound. On the other hand, the reaction temperature should not be so high as to cause the decomposition to occur too quickly and for the sulfur to be lost before the reaction occurs. In this latter situation, when a high temperature is used, the level and number of impurities, caused for example by the over-reduction of the transition metal ion to the zero oxidation or metallic state, is also found to increase. Therefore, the actual temperature used will depend on the chosen starting materials: at least one transition metal sulfide and a lithium-containing compound. As a general rule, the reaction temperature is conveniently from 500 to 1500° C., preferably from 550 to 1500° C., further preferably from 550 to 950° C. and particularly preferably from 550 to 750° C. The reaction time varies according to reaction temperature and as one might expect, the higher the temperature, the faster the reaction. By way of example, a suitable reaction temperature/time profile that will produce the desired lithium-containing transition metal sulfide includes heating the reaction mixture for 12 hours at 650° C. Alternatively, one could heat the reaction mixture for 4 hours at 950° C.

The at least one transition metal sulfide used in the method of the first patent application may be one or more sulfide compounds comprising one or more transition metals. This includes the use of single and/or mixtures of several transition metals in the sulfide, as well as the use of mono- and/or di-sulfides. Particularly suitable transition metals comprise one or more of manganese, iron, cobalt, nickel copper and zinc. Preferably, the transition metals are selected from manganese, iron, cobalt and nickel. Sulfides that comprise iron are the most preferred transition metal sulfides.

Although the starting materials are not air or moisture sensitive, and these positive attributes aid the storage and handling of these materials, the reaction product is itself reactive towards water. Therefore, it is advantageous to form and handle the lithium-containing transition metal sulfides under a dry and inert atmosphere such as argon or nitrogen.

Suitable reaction vessels comprise glassy carbon or graphite crucibles that generally have a loose fitting lid. However, a sealed pressurized vessel may also be used. For commercial scale production, it is advantageous to use a continuous process, for example, a rotary tube furnace, although a retort batch process may also be used.

The reaction described in the first patent application is a solid-state reaction and this means that all reactants are in solid form and are without the use of a reaction medium such as a solvent. The reactants are solid materials that are first grounded using a ball mill to produce a fine powder that can either be used directly or pressed into a pellet.

As will be explained below, the Applicants have found surprising benefits in the properties of the reaction product when the method of the first patent application involves heating the starting materials in the presence of carbon—this reaction process may be termed a “carbon assisted” process. Carbon, as with the other means of providing reducing conditions, is useful to reduce the lithium sulfate (Li₂SO₄), which forms as a side reaction during the reaction process, to lithium sulfide (Li₂S), which in turn reacts with the transition metal sulfide to form the desired lithium-containing transition metal sulfide. It is highly desirable that the reducing conditions in the process do not directly reduce the oxidation state of the transition metal ion. Any amount of carbon may be used but it is convenient not to use too much to prevent it from becoming a significant impurity in the reaction product. Having said this, it has been found to be of significant advantage, particularly to the conductivity of the target material, for at least a small amount of carbon to be present in the reaction product. Moreover, there are further specific advantages to be gained in the carbon being residual from the carbon assisted reaction process, as opposed to it merely being added later to a sample of the target lithium-containing transition metal sulfide material. During the carbon assisted process, the carbon is considerably intimately mixed with the lithium-containing transition metal sulfide product. The degree of mixing described as “intimate” refers specifically to the chemical as opposed to physical mixing that is achieved when carbon is used, at least in part, to provide the reducing conditions in the process of the first patent application. This “intimate” mixing is quite different from the degree of mixing that would ever be achieved using ball milling or other physical mixing apparatus. In particular, in the carbon-assisted process, the carbon is dispersed at the microscopic level on individual particles of the lithium-containing transition metal sulfide.

SUMMARY

Thus the present invention provides a composition comprising at least one lithium-containing transition metal sulfide and carbon wherein particles of the carbon are dispersed at the microscopic level on individual particles of the at least one lithium-containing transition metal sulfide.

The ratio of reaction starting materials used in the method of the first patent application is typically 1 mole of transition metal sulfide:the equivalent of from 0.5 to 4 moles of lithium ion in the lithium containing material:from 0 to 5 moles of the one or more reducing agent. The preferred ratio of starting materials, that is transition metal sulfide:number of mole equivalents of lithium supplied by the lithium-containing compound:number of moles of one or more reducing agent, is 1:0.5-2:0.25-5, further preferably 1:0.5-1:0.25-0.5. In the case of the carbon assisted process, i.e. the process which may be used to make the compositions of the present invention, the most preferred ratio of reactants, that is, transition metal sulfide: number of mole equivalents of lithium supplied by the lithium-containing compound:carbon, is 1:1:0.5.

As a general rule, lower amounts of carbon are required when a reducing gas and/or a reducing agent is used.

The carbon used may be in any suitable form, for example, graphite, charcoal and carbon black, although it is preferred to use high surface area carbons that are typically used in electrode formulations, for example Super P, Denka Black, Ensaco etc.

An alternative source of carbon may be derived in situ from any suitable carbonaceous material, for example, by the thermal decomposition of an organic material such as lithium acetate, dextrin, starch, flour cellulosic substance or sucrose or a polymeric material such as polyethylene, polyethylene glycol, polyethylene oxide, and ethylene propylene rubber. In fact, most carbon containing materials may be used, provided their thermal decomposition does not involve the production of detrimental by-products.

The target lithium-containing transition metal sulfide compound produced by the method of the first patent application is generally found to contain less than 2 atoms of lithium per molecule of product. Even though 1.72 atoms of lithium is the typical number, the true number will depend on the reaction temperature and duration of the heating step and the particular lithium-containing transition metal sulfide being prepared.

In order to reduce the quantity of impurities formed and to optimise the reaction conditions of the method of the first patent application, it has been found advantageous to add a flux agent, also known as a mineraliser, to the reaction mixture. Flux agents or mineralisers are commonly used in the ceramics industry to lower the reaction temperature and shorten reaction times. Mineralisers such as sodium chloride, borax, lithium chloride, lithium fluoride, sodium fluoride, lithium borate and sodium carbonate are known. If a very small amount of a mineraliser, in particular an alkali metal halide, is used, this will result in a lithium-containing transition metal sulfide product that exhibits enhanced crystallinity with lower levels of impurities. Any alkali metal halide may be used but lithium chloride and lithium iodide are most preferred. Alternatively, sodium carbonate or sodium chloride may be used. However in this case, it is likely that at least some substitution of the lithium for sodium will occur in the target product. The amount of mineraliser found to be beneficial is from 1 to 5% by weight of the starting materials, preferably from 1 to 3% by weight and further preferably 1% by weight of the starting materials.

A typical electrode comprises 94% of a lithium-containing material, 3% of a binder and 3% of a carbon-containing material. The lithium-containing material is preferably a composition comprising at least one lithium-containing transition metal sulfide and carbon wherein the particles of the carbon are dispersed at the microscopic level on individual particles of the lithium-containing transition metal sulfide. Further preferably, the lithium-containing transition metal sulfide has been made using the carbon-assisted process described above in relation to the method according to the first patent application. The binder can be any material known in the art to be suitable for use as a binder, usually a highly inert polymer such as polytetrafluoroethylene (PTFE), polymers of ethylene propylene diamine monomer (EPDM), polyethylene oxide (PEO), polyacrylonitrile and polyvinylidene fluoride. The Applicant's preferred binder is ethylene propylene diene monomer (EPDM). The key feature of the binder is that it needs to be able to form a slurry or paste with the lithium-containing transition metal sulfide, which in turn may be coated onto a current collector. Conveniently, mixing the binder with a solvent facilitates coating. Any solvent may be used, provided it is nonpolar, dry and does not react with either the binder or the lithium-containing transition metal sulfide. Desirably, the solvent is reasonably volatile to facilitate its removal at room temperature. Suitable solvents might include low molecular weight halogenated compounds, particularly halogenated hydrocarbons such as methylene chloride or low molecular weight materials, such as cyclohexane, trimethylbenzene (TMB), toluene, and xylene, or low molecular weight alcohols, such as methanol and mixtures of any of these compounds. Trimethylbenzene is a preferred solvent.

The binder/solvent/lithium-containing transition metal sulfide slurry/paste may also include additives adapted to modify the properties of the binder. The chosen additive must be naturally compatible with the binder, the lithium-containing transition metal sulfide and the electrolyte, and must not affect the performance of the finished cell.

Compositions according to the present invention that contain at least one lithium-containing transition metal sulfide and carbon and that are preferably produced by the method of the first patent application, are useful in a wide variety of applications where a low voltage rechargeable battery power source may be used, for example, in mobile phones, vehicles, lap top computers, computer games, cameras, personal CD and DVD players, drills, screw drivers and flash lights and other hand-held tools and appliances.

In order for the lithium-containing transition metal sulfide/carbon compositions of the present invention to be used in such applications it is necessary to construct them into an electrochemical cell. Different methods of making such cells are described in the literature, but one particularly convenient example is described in EP 1 295 355 B1. In this case, an electrochemical cell is assembled comprising a plurality of anode plates and a plurality of cathode plates, each comprising respective insertion materials, for example, graphite in the anode plates and the lithium-containing transition metal sulfides/carbon compositions of the present invention in the cathode plates. In particular, the method involves forming a stack of discrete, separate cathode plates and discrete, separate anode plates stacked alternately, each comprising a layer of a respective ion insertion material bonded to a metal current collector, and interleaving a continuous separator/electrolyte layer between successive plates so it forms a zigzag.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be particularly described by way of example with reference to the following drawings in which:

FIG. 1 shows the overall reaction scheme for the process of the first patent application in which lithium carbonate is used as the lithium-containing compound. The reaction proceeds by the thermal decomposition of lithium carbonate to lithium oxide followed by the reaction of the latter with sulfur evolved from the transition metal sulfide and carbon to produce lithium sulfide, which in turn reacts with the transition metal sulfide to produce the target lithium-containing transition metal sulfide composition that comprises particles of carbon dispersed at the microscopic level on individual particles of the lithium transition metal sulfide.

FIG. 2 shows multiple constant current cycling data for the compound with the formula Li_(2-x)FeS₂ made according to Example 1, which produces compositions according to the present invention.

FIG. 3 shows multiple constant current cycling data for the compound with the formula Li_(2-x)FeS₂ made according to Example 2, which produces compositions according to the present invention.

FIG. 4 shows a series of overlaid X-ray diffraction spectra to show the effect on the purity and crystal structure of the addition of 0%/wt, 1%/wt and 3%/wt lithium chloride mineraliser in compounds with the formula Li_(2-x-y) A_(y) Fe_(1-z) M_(z)S₂ made according to Examples 1, 2 and 3 respectively.

FIG. 5 shows a series of overlaid X-ray diffraction spectra to show the effect of carbon on the purity of Li_(2-x)FeS₂ compounds in Examples 1 and 4.

DESCRIPTION OF THE EMBODIMENTS General Laboratory Scale Method a) For Making Compounds with the Formula Li_(2-x-y) A_(y) Fe_(1-z) M_(z)S₂Using a Reducing Agent to Provide the Reducing Conditions

The lithium containing compound, transition metal sulfide and reducing agent are weighed out into a ball mill pot. This mixture is milled for 1-12 hrs depending on the size of the precursor mix at a rate of 200-350 rpm. The precursor mix is then pelletized and placed into a glassy carbon crucible with a lid. The carbon crucible is placed into the furnace under a gentle flow of an inert gas, and heated between 500 and 1500° C. at a rate of 1 to 10° C. per minute over a period of 1 to 12 hours. The crucible is allowed to cool under the inert gas flow and transferred directly into a glove box. The resulting product is ground initially using a pestle and mortar and then milled more finely using a ball mill. The lithium-containing transition metal sulfide product may then be analysed using X-ray diffraction and/or electrochemical techniques. A suitable furnace for carrying out the above process may be a graphite lined rotary furnace, a retort furnace or a static tube furnace.

General Laboratory Scale Method b) For Making Compounds with the Formula Li_(2-x-y) A_(y) Fe_(1-z) M_(z)S₂Using a Reducing Gas to Provide the Reducing Conditions

The lithium-containing compound and transition metal sulfide are weighed out into a ball mill pot, this mixture is milled for 1-12 hrs depending on the size of the precursor mix at a rate of 200-350 rpm. The precursor mix is then pelletized and placed into a glassy carbon crucible with a lid. The carbon crucible is placed into the furnace under a gentle flow of a reducing gas, and heated between 500 and 800° C. for 1 -20 hours dwell. The crucible is allowed to cool under the inert gas flow and transferred directly into a glove box and processed and analysed as described above.

Compounds with the Formula Li_(2-x-y) A_(y) Fe_(1-z) M_(z)S₂ y and z are as defined above) were prepared according to Examples 1 to 8 summarised in Table 1 below:

TABLE 1 Li- Transition Reaction containing metal Reducing conditions Example compound sulfide conditions Mineraliser Temp/Time 1 Li₂CO₃ FeS₂ Carbon None 135° C. 2 hr (111 g; (179 g; Denka Black 900° C. 2 hrs 1.5 moles) 1.5 moles) (9 g; 3° min⁻¹ 0.75 moles) 2 Li₂CO₃ FeS₂ Carbon LiCl 135° C. 2 hr (111 g; (179 g; Denka Black (2 g; 1 wt %) 750° C. 9 hrs 1.5 moles) 1.5 moles) (9 g; 3° min⁻¹ 0.75 moles) 3 Li₂CO₃ FeS₂ Carbon LiCl 135° C. 2 hr (111 g; (179 g; Denka Black (6 g; 3 wt %) 750° C. 9 hrs 1.5 moles) 1.5 moles) (9 g; 3° min⁻¹ 0.75 moles) 4 Li₂CO₃ FeS₂ None None 135° C. 2 hr comparison (5.53 g; (8.99 g; 750° C. 9 hrs 0.075 moles) 0.075 moles) 3° min⁻¹ 5 Li₂CO₃ FeS₂ Carbon Na₂CO₃ 135° C. 2 hr (111 g; (179 g; Denka Black (0.79 g; 0.01 moles), 750° C. 9 hrs 1.5 moles), 1.5 moles) (9 g; LiCl (2 g; 3° min⁻¹ 0.75 moles) 0.05 moles) 6 Li₂CO₃ CoS₂ Carbon None 135° C. 2 hr (111 g; (179 g; Denka Black 800° C. 9 hrs 1.5 moles) 1.5 moles) (9 g; 3° min⁻¹ 0.75 moles) 7 Li₂CO₃ FeS₂ H_(2/)N₂ mixture None 500-800° C. comparison (73.89 g; (119.98 g; for 1-20 1 mole) 1 mole) hours dwell 8 Li₂CO₃ FeS₂ Carbon None 500-800° C. comparison (73.89 g; (119.98 g; monoxide for 1-20 1 mole) 1 mole) hours dwell

General Procedure to Determine the Capacity of Li_(2-x-y) A_(y) Fe_(1-z)M_(z)S₂ Compounds

Materials were initially tested using a small pouch type cell, tags of aluminium one side and nickel on the opposing side were sealed into the sides of a pouch, a stack of Li/Ni separator and the cathode coating was made and inserted into the pouch in between the two tags. Electrolyte was pipetted onto the separator and the end of the pouch was then vacuum-sealed. Constant current tests were performed on a MACCOR between the voltage limits 2.65V and 1.45V using a rate of 10 mAg⁻¹.

Determination of X-Ray Diffraction Data

Powder X-ray diffraction data was obtained using a SIEMANS D5000 using a copper Kα₁ and Kα₂ source. The sample was placed into an air sensitive holder, which consisted of a Perspex dome which sealed over the sample, thus preventing degradation of the material during data collection. Phase analysis data were collected over a period of 4 hours 10-80 °2 theta, whilst high quality data were collected 10-90°2 theta over a period of 13 hours.

Determination of the Capacity of the Li_(2-x-y) A_(y) Fe_(1-z) M_(z)S₂ Compound Made Using Example 1

The major phase present in the product synthesised in Example 1 was found to be Li_(2-x)FeS₂, with low levels of impurities observed by x-ray diffraction. As shown in FIG. 2, constant current data was obtained for the product using the above general testing procedure. In particular, multiple constant current cycling data was obtained using 1.2M LiPF6 in EC:EMC 20:80 as the electrolyte and cycling against a lithium metal anode between the voltage limits 2.65V and 1.45V at room temp at a current of 10 mAg⁻¹. An initial charge capacity of 290 mAhg⁻¹ versus lithium was observed. A reversible capacity of 320 mAhg⁻¹ was observed over the subsequent cycles.

Investigation of the Effect of Adding 1%/wt Mineraliser During the Preparation of Li_(2-x)FeS₂

The Li_(2-x)FeS₂ compound made according to Example 2 includes 1%/wt LiCl mineraliser in its formulation. The effect of this mineraliser on the capacity of Li_(2-x)FeS₂ is indicated by constant current cycling data shown in FIG. 3. The cycling data was obtained using the above general procedure, using 1.2M LiPF6 in EC:EMC 20:80 as the electrolyte and cycling against a lithium metal anode between the voltage limits 2.65V and 1.45V at room temp at a current of 10mAg⁻¹. An initial charge capacity of 290 mAhg⁻¹ and a discharge capacity of 320 mAhg⁻¹ versus lithium was observed. A reversible capacity of 320 mAhg⁻¹ is observed over the subsequent 6 cycles. Also, a slight reduction in polarisation was observed for the sample made using a lithium chloride mineraliser compared to the sample synthesised with none, as shown in FIG. 2.

Comparison of the Effect of Adding 0 wt %, 1 wt % Versus 3 wt % Mineraliser During the Preparation of Li_(2-x)FeS₂

The Li_(2-x)FeS₂ compounds made according to Examples 1, 2 and 3 involved the use of 0 wt %, 1wt % and 3wt % respectively of lithium chloride as a mineraliser.

FIG. 4 shows a pattern at the bottom of the three graphs that depicts the calculated positions for the reflections associated with Li_(2-x)FeS₂. As the level of LiCl additive increases the reflection peaks for Li_(2-x)FeS₂ appear sharper, which suggests a larger particle size, and less impurity peaks appear in the 20°2 theta region. A small level of LiCl is observed as impurity in the 3w % LiCl containing-material, and capacity data for the 3wt % LiCl-containing material is reduced due to the LiCl being present as an impurity. Such reduction in capacity is not observed for the Li_(2-x)FeS₂ compound made with 1wt % LiCl.

Determination of the Effect of Carbon Levels on the Purity of Li_(2-x)FeS₂ Compounds

The Li_(2-x)FeS₂ compound of Example 4 was made without carbon in an argon atmosphere, and X-Ray diffraction data was obtained using the general procedure outlined above. Similar X-ray data was obtained for the compound of Example 1 and both sets of results are illustrated in FIG. 5. The peaks at the bottom of the graphs show the calculated position for Li₂FeS₂. The sample containing iron sulfide, the amount of lithium ion in lithium carbonate, and carbon in the ratio 1:1:0.5 (Example 1), shows low impurity levels compared to a similar compound but with no carbon (Example 4). The sample made with no carbon shows impurities of Li₂SO₄ and FeS depicted by ⋄ and X respectively above the reflection peaks. These results show that Li_(2-x)FeS₂ compounds can be made with no carbon being present, however impurity phases of Li2SO₄ and FeS are present. Thus a key feature of the reducing agent (for example the carbon) is to reduce the Li₂SO₄ to Li₂S, such that the remaining FeS can react with lithium sulfide to form the target Li_(2-x)FeS₂ compound as detailed in the reaction scheme in FIG. 1. 

1. A composition comprising at least one lithium-containing transition metal sulfide and carbon wherein particles of the carbon are dispersed at the microscopic level on individual particles of the lithium-containing transition metal sulfide.
 2. A composition comprising at least one lithium-containing transition metal sulfide and carbon wherein particles of the carbon are dispersed at the microscopic level on individual particles of the lithium-containing transition metal sulfide, and further wherein the dispersion of particles of carbon is effected during the synthesis of the lithium-containing transition metal sulfide.
 3. A composition according to claim 1 wherein the at least one lithium-containing transition metal sulfide is of the formula Li_(2-x-y) A_(y) Fe_(1-z) M_(z)S₂ where x=0 to 1.5, y=0 to 1; z=0 to 1, A is selected from one or more of silver (Ag), sodium (Na), copper (Cu(I)) and potassium (K) and M is a generic representation for one or more transition metals.
 4. A composition according to claim 1 wherein the transition metal in the at least one lithium-containing transition metal sulfide comprises one or more of manganese, iron, cobalt, nickel, copper and zinc.
 5. A composition according to claim 3 wherein the transition metal in the at least one lithium-containing transition metal sulfide comprises one or more of manganese, iron, cobalt, nickel, copper and zinc.
 6. A composition according to claim 4 wherein the transition metal comprises one or more of manganese, iron, cobalt and nickel.
 7. A composition according to claim 5 wherein the transition metal comprises one or more of manganese, iron, cobalt and nickel.
 8. A composition comprising at least one lithium-containing transition metal sulfide and carbon wherein the at least one lithium-containing transition metal sulfide is of the formula Li_(2-x-y) A_(y) Fe S₂ where x=0 to 1.5, y=0 to 1, and A is selected from one or more of silver (Ag), sodium (Na), copper (Cu(I)) and potassium (K).
 9. A composition according to claim 8, wherein the particles of carbon are derived from any carbonaceous material suitable to provide a source of carbon when heated.
 10. A composition according to claim 8 in powder form.
 11. A composition according to claim 8, further comprising one or more mineralisers.
 12. A composition according to claim 11 wherein the one or more mineralisers comprise an alkali metal halide.
 13. Use of the composition according to claim 1 in the preparation of an electrode.
 14. Use of the composition according claim 1 in the preparation of a cathode.
 15. An electrode according to claim 12, further comprising a binder that is used in conjunction with a solvent to form a slurry or paste with the lithium-containing transition metal sulfide-carbon composition.
 16. An electrode according to claim 14 wherein the one or more solvents comprise a non-polar hydrocarbon solvent.
 17. An electrode according to either of claim 14 in conjunction with a counter electrode and an electrolyte, in a lithium ion battery.
 18. An electrode according to claim 14 in conjunction with a counter electrode and an electrolyte, in a low voltage rechargeable battery power source.
 19. A lithium ion battery comprising a cathode comprising a lithium-containing transition metal sulfide—carbon composition according to claim
 1. 20. A lithium ion battery comprising a cathode comprising a lithium-containing transition metal sulfide—carbon composition according to claim
 3. 